Purifying agent and purification method for halogen-containing exhaust gas

ABSTRACT

A cleaning agent and a cleaning process for efficiently removing noxious halogen-based gases such as fluorine, chlorine, boron trifluoride, boron trichloride and tungsten hexafluoride from exhaust gases from semiconductor fabrication processes. The cleaning agent is produced by adherently adding alkali metal formate and/or alkaline earth metal formate to activated carbon, or adherently adding alkali metal hydroxide and/or alkaline earth metal hydroxide together with alkali metal formate and/or alkaline earth metal formate to activated carbon. By exposing exhaust gases to the cleaning agent, noxious halogen-based gases in the exhaust gases are efficiently removed with little desorption of halogen-based gases adsorbed on the cleaning agent. Also, the cleaning treatment is further improved in safety and efficiency by a pre-treatment cleaning agent comprising a metal oxide or a metal hydroxide and a post-treatment cleaning agent prepared by adherently adding sodium formate to a metal oxide.

TECHNICAL FIELD

The present invention relates to cleaning agents for treating exhaustgases containing harmful halogen-based gases and processes for cleaningthe exhaust gases, and more particularly, to cleaning agents fortreating halogen-containing exhaust gases discharged from dry etchingprocess of semiconductor fabrication for selectively removing thin filmon wafer in predetermined patterns, and cleaning processes for theexhaust gases. In particular, the present invention relates to drycleaning agents and dry cleaning processes.

BACKGROUND ART

Known cleaning processes for halogen-containing exhaust gases have beengenerally classified into wet and dry processes. The typical wet processusing an aqueous alkali solution as an absorbing solution is anexcellent method due to its large treating capacity and wideapplicability. However, the wet cleaning process has drawbacks such aslow cleaning efficiency, incomplete cleaning of halogen-containingexhaust gases, need for time- consuming maintenance of cleaningapparatus, danger of contamination of products by back streaming waterfrom absorbing solution into semiconductor fabrication apparatus, etc.For these reasons, the wet processes are not used for cleaning exhaustgases from dry etching process. Therefore, the cleaning of exhaust gasesfrom dry etching process have been preformed by a dry cleaning processutilizing physical adsorption or a dry cleaning process in whichhalogen-based gases are fixed by chemical reactions.

In semiconductor fabrication industries, dry etching techniques havebeen used for selectively removing parts of silicon dioxide thin films,polysilicon thin films, tungsten thin films or aluminum thin films inpredetermined patterns. The etching gases for dry etching areappropriately selected depending on natures of thin films to be etched,and are required to have a high reactivity with thin films and producereaction products readily volatilizable from thin films. To meet theserequirements, halogen-based gases have been widely used as the etchinggases, so that exhaust gases from dry etching process inevitably containvarious kinds of halogen-based gases according to kinds of halogen-basedetching gases used, kinds of thin films to be etched and etchingconditions. Therefore, the treatment of exhaust gases requires drycleaning agents and dry cleaning techniques according to kinds andproperties of individual halogen-based gases contained in exhaust gases.The halogen-based gases contained in exhaust gases from dry etchingprocess are generally classified into fluorine-based gases andchlorine-based gases. The fluorine-based gases include fluorine gas, andthe chlorine-based gases include chlorine gas. Differences in chemicalproperties and reactivity between fluorine gas and chlorine gas areimportant in designing dry cleaning agents and dry cleaning processes.

In dry etching processes, in general, a halogen-based gas is fed into achamber during etching, and an inert gas is fed after the etching. As aresult, an exhaust gas containing the halogen-based gas and an inert gascontaining a little or no halogen-based gas enter repeatedly into anexhaust gas cleaning apparatus.

Hitherto, in dry cleaning of fluorine-containing exhaust gases, acleaning agent utilizing physical adsorptivity of activated carbons hasbeen used. Such a cleaning agent can be produced at a low cost, and canremove a relatively large amount of fluorine gas and fluorine compoundgases with a small amount of use. However, fluorine gas contained inexhaust gases, in some cases, explosively reacts with activated carbon.In fact, there have been reported many explosion accidents due toexplosive reaction.

To avoid the above problems, a cleaning agent which is composed mainlyof a metal hydroxide to utilize its chemical reactivity with fluorinehas been employed for dry-cleaning fluorine-containing exhaust gases.For example, Japanese Patent Application Laid-Open No. 9-99216 teaches acleaning agent for acidic gases, which is composed mainly of strontiumhydroxide. The strontium hydroxide-based cleaning agent for acidic gasescan be produced at low cost and can remove a relatively large amount offluorine gas and various fluorine compound gases with a small amount ofuse. In addition, unlike activated carbon, the cleaning agent can besafely used without any risk of explosive reaction.

Contrary to the dry cleaning of fluorine-containing exhaust gases, inthe dry cleaning of chlorine-containing exhaust gases, a cleaning agentutilizing physical adsorptivity of activated carbon is usable withoutany risk of explosive reaction, thereby enabling production of thecleaning agent at low cost.

In general, activated carbon, when used as a cleaning agent forhalogen-containing exhaust gases, shows a relatively large cleaningcapability in treatment of continuously flowing exhaust gases. However,the cleaning capability of activated carbon is disadvantageouslydecreased in treating alternative flow of an exhaust gas containinghalogen-based gas and a gas (purge gas) containing no halogen-based gasas experienced in the treatment of exhaust gases from etching process ofsemiconductor production. This is because the halogen-based gasphysically adsorbed on the activated carbon is gradually desorbed bylong-term flowing of a gas containing no halogen-based gas. To eliminatethis problem, various studies have been made to enhance the cleaningcapability of activated carbon by adherently adding various chemicalagents to activated carbon and fixing adsorbed halogen-based gas toactivated carbon by chemical reaction between the chemical agents andthe adsorbed halogen-based gas, thereby preventing the desorption ofadsorbed halogen-based gas. For example, Japanese Patent ApplicationLaid-Open No. 58-122025 proposes a method to adherently add alkali metalhalides to activated carbon, and Japanese Patent Application Laid-OpenNo. 4-210236 proposes a method to adherently add alkali metal aluminateor tetraalkylammonium aluminate to activated carbon. However, any ofthese methods fail to exhibit satisfactory effects.

Also, various cleaning agents using no activated carbon and capable offixing halogen-based gas by chemical reaction have been developed. Forexample, Japanese Patent Application Laid-Open No. 9-234337 proposes acleaning agent prepared by adherently adding sodium formate to a metaloxide composed mainly of copper oxide and manganese oxide. The proposedcleaning agent is very excellent since it can completely fixhalogen-based gases, but requires high production cost and is relativelylow in cleaning capability as compared to activated carbon-basedcleaning agents.

Further, a cleaning agent composed mainly of strontium hydroxide andtri-iron tetroxide (Japanese Patent Application Laid-Open No. 7-308538)shows high cleaning capability in treating exhaust gases containinghalogen-based gas in high content. However, when the content ofhalogen-based gas is low, the cleaning capability is inferior to that ofthe cleaning agent prepared by adherently adding sodium formate to ametal oxide composed mainly of copper oxide and manganese oxide(Japanese Patent Application Laid-Open No. 9-234337). In addition, thecleaning agent loses water by evaporation during the use, resulting indecrease of the cleaning capability due to drying.

DISCLOSURE OF THE INVENTION

Accordingly, it is an object of the present invention to develop acleaning agent for dry-cleaning halogen-containing exhaust gases, whichis produced at low cost, which has a high cleaning capability and whichis safely used even in the cleaning of fluorine-containing exhaustgases. It is another object of the present invention to provide acleaning agent which is free from desorption of adsorbed halogen-basedgases even when the cleaning agent is exposed to the flow of ahalogen-free gas for a long period of time after the cleaning treatment,and a cleaning method using the cleaning agent.

As a result of extensive researches for solving the above problems, thepresent inventors have found that halogen-based gases are efficientlyremoved from halogen-containing exhaust gases by contacting thehalogen-containing exhaust gas with a cleaning agent prepared byadherently adding alkali metal formate and/or alkaline earth metalformate to activated carbon, or a cleaning agent prepared by adherentlyadding alkali metal hydroxide and/or alkaline earth metal hydroxide inaddition to alkali metal formate and/or alkaline earth metal formate toactivated carbon. It has been also found that even when exhaust gases tobe treated contain fluorine gas or chlorine trifluoride, the cleaningtreatment is safely performed by disposing a cleaning agent composed ofmetal oxide or metal hydroxide on the upstream of the cleaning agent. Ithas been further found that halogen-based gases desorbed from activatedcarbon are securely captured by disposing a cleaning agent prepared byadherently adding sodium formate to a metal oxide composed mainly ofcopper oxide and manganese oxide on the downstream of the cleaningagent. The present invention has been accomplished based on thesefindings.

Thus, in a first aspect of the present invention, there is provided acleaning agent for halogen-containing exhaust gases, which comprisesactivated carbon adherently added with alkali metal formate and/oralkaline earth metal formate, an adhered amount of the alkali metalformate and/or alkaline earth metal formate being 3 to 18% by weight(dry basis) based on the weight of the cleaning agent.

In a second aspect of the present invention, there is provided acleaning agent for halogen-containing exhaust gases, which comprisesactivated carbon adherently added with alkali metal hydroxide and/oralkaline earth metal hydroxide in addition to alkali metal formateand/or alkaline earth metal formate, a total adhered amount of thealkali metal formate and/or alkaline earth metal formate and the alkalimetal hydroxide and/or alkaline earth metal hydroxide being 3 to 18% byweight (dry basis) based on the weight of the cleaning agent.

In a third aspect of the present invention, there is provided a processfor cleaning halogen-containing exhaust gases, which comprisescontacting an exhaust gas containing halogen-based gas as noxiouscomponent with a cleaning agent comprising activated carbon adherentlyadded with alkali metal formate and/or alkaline earth metal formate.

In a fourth aspect of the present invention, there is provided a processfor cleaning halogen-containing exhaust gases, which comprisescontacting an exhaust gas containing halogen-based gas as noxiouscomponent with a cleaning agent comprising activated carbon adherentlyadded with alkali metal hydroxide and/or alkaline earth metal hydroxidein addition to alkali metal formate and/or alkaline earth metal formate.

BEST MODE FOR CARRYING OUT THE INVENTION

The cleaning agent and the cleaning process of the present invention ismainly applied to the treatment of halogen-containing exhaust gases frometching process of semiconductor production. The cleaning agentaccording to the present invention is produced by adherently addingalkali metal formate and/or alkaline earth metal formate, and optionallyalkali metal hydroxide and/or alkaline earth metal hydroxide toactivated carbon.

The activated carbon usable in the present invention is not particularlyrestricted as long as it has high adsorptivity and particle sizessuitable for use as cleaning agents. The activated carbon usually has aspecific surface area of 700 to 2,500 m²/g, preferably 1,000 to 2,500m²/g, and a particle size of passing through 32 to 4 mesh sieves.Examples of suitable activated carbons include coal activated carbon,charcoal activated carbon, coconut shell activated carbon, etc.Preferred is coconut shell activated carbon.

Examples of preferred alkali metal formates include sodium formate andpotassium formate. Examples of preferred alkaline earth metal formatesinclude calcium formate, magnesium formate, barium formate and strontiumformate. These formates may be used alone or in combination of two ormore. Sodium formate and potassium formate are especially preferred dueto their low cost and easy availability.

The alkali metal formate and/or alkaline earth metal formate may beadhered to the activated carbon by any methods as long as the formatesare uniformly adhered on the activated carbon. For example, there may beused a method of impregnating activated carbon with an aqueous solutionof the alkali metal formate and/or alkaline earth metal formate and thendrying the impregnated activated carbon, or a method of sprinkling theaqueous solution over activated carbon while stirring and then dryingthe sprinkled activated carbon.

The adhered amount of the alkali metal formate and/or alkaline earthmetal formate on the activated carbon is 3 to 18%, preferably 5 to 15%by weight based on the weight of dry cleaning agent.

The cleaning agent of the present invention obtained by adherentlyadding the alkali metal formate and/or alkaline earth metal formate tothe activated carbon is considerably reduced in the desorption amount ofhalogen-based gas when exposed to the flow of a halogen-free gas afterthe cleaning treatment of halogen-containing exhaust gases. When theconventional activated carbons or the conventional cleaning agentsprepared by adherently adding various chemical agents to activatedcarbon are used in the cleaning treatment of halogen-containing exhaustgases, adsorbed halogen-based gas is easily desorbed upon exposure tothe flow of a halogen-free gas after the cleaning treatment. On thecontrary, the cleaning agent according to the present invention isreduced in the desorption amount of halogen-based gas even when exposedto the flow of a halogen-free gas, and therefore, considerably improvedin cleaning capability as compared to the conventional activated carbonsand cleaning agents adhered with chemical agents.

In the cleaning agent of the present invention, the activated carbon maybe further adhered with alkali metal hydroxide and/or alkaline earthmetal hydroxide in addition to the alkali metal formate and/or alkalineearth metal formate. With such an additional adherent component, thecleaning agent is further reduced in the desorption amount ofhalogen-based gas, thereby further increasing cleaning capability ascompared to cleaning agents adherently added with only formates.

Examples of the alkali metal hydroxides and alkaline earth metalhydroxides include sodium hydroxide, potassium hydroxide, lithiumhydroxide, calcium hydroxide, barium hydroxide, strontium hydroxide andmagnesium hydroxide. These hydroxides may be used alone or incombination of two or more. Of these hydroxides, sodium hydroxide andpotassium hydroxide are especially preferred in view of their low costand easy availability.

When both the formate (alkali metal formate and/or alkaline earth metalformate) and the hydroxide (alkali metal hydroxide and/or alkaline earthmetal hydroxide) are adherently added to activated carbon, thequantitative proportion of these compounds is not particularlyrestricted, and the equivalent ratio of the formate to the hydroxide isusually 1:0.1-3.0, preferably 1:0.5-2.0, more preferably 1:0.75-1.5. Thecleaning agents comprising activated carbon adherently added with boththe formate and the hydroxide may be produced by a method ofimpregnating activated carbon with an aqueous solution containing theformate and the hydroxide in the above equivalent ratio range and thendrying the impregnated activated carbon, or a method of sprinkling theaqueous solution to activated carbon under stirring and then drying thesprinkled activated carbon. Alternatively, the cleaning agent may beproduced by first adherently adding one of the formate and the hydroxideto the activated carbon and then adherently adding the other.

The total adhered amount of the alkali metal formate and/or alkalineearth metal formate and the alkali metal hydroxide and/or alkaline earthmetal hydroxide is 3 to 18% by weight, preferably 5 to 15% by weightbased on dried cleaning agent.

In the present invention, the halogen-containing exhaust gases areusually cleaned by passing the halogen-containing exhaust gases througha cylindrical cleaning column packed with a cleaning agent. The cleaningtreatment is usually carried out at ordinary temperatures withoutnecessity of additional heating or cooling. The concentration of thehalogen-based gas in the exhaust gases to be treated is usually 5% byvolume or lower. Although the contact time between the cleaning agentand the exhaust gas is not specifically restricted because it variesdepending upon the concentration of the halogen-based gas, the contacttime is usually 5 cm/sec or less in terms of superficial linear velocity(LV). The cleaning pressure is not particularly restricted, but thecleaning treatment is usually carried out under atmospheric pressure.

After treating the exhaust gas with the cleaning agent of the presentinvention, the exhaust gas may be further contacted with apost-treatment cleaning agent capable of fixing the halogen-based gas bychemical reaction. With this post-treatment, the treated gases can bealways completely prevented from being contaminated with desorbedhalogen-based gas.

As the post-treatment cleaning agent, a cleaning agent, for example,prepared by adherently adding sodium formate to a metal oxide asdescribed in Japanese Patent Application Laid-Open No. 9-234337 ispreferably used. The metal oxide is composed mainly of copper oxide andmanganese oxide, and may be a mixture further containing another metaloxide such as silver oxide, aluminum oxide, silicon oxide, potassiumoxide and sodium oxide. The total amount of copper oxide (II) andmanganese oxide (IV) in the mixture is usually 60% by weight or more,and the weight ratio of manganese oxide (IV) to copper oxide (II) isusually 1:0.2-5.0. Such a mixture is commercially available as“hopcalite”. The adhered amount of sodium formate is usually 1 to 60parts by weight based on 100 parts by weight of metal oxides.

The post-treatment is usually performed by contacting the exhaust gaswith the post-treatment cleaning agent at ordinary temperature and asuperficial linear velocity of 5 cm/sec or less under atmosphericpressure.

The cleaning method using the cleaning agent of the present inventionalone and the cleaning method using both the cleaning agent of thepresent invention and the post-treatment cleaning agent are especiallyeffective for cleaning the exhaust gases containing at least one maingas selected from the group consisting of nitrogen, argon, helium andhydrogen, and at least one halogen-based gas selected from the groupconsisting of chlorine, bromine, iodine, hydrogen fluoride, hydrogenchloride, hydrogen bromide, hydrogen iodide, boron trifluoride, borontrichloride, silicon tetrafluoride, silicon tetrachloride, titaniumtetrachloride, aluminum chloride, germanium tetrafluoride and tungstenhexafluoride.

Before treating the halogen-containing exhaust gases with the cleaningagent of the present invention, the exhaust gases may be contacted witha pre-treatment cleaning agent to remove in advance fluorine or chlorinetrifluoride which tends to violently react with activated carbon. Thepre-treatment may be usually carried out by contacting the exhaust gaseswith the pre-treatment cleaning agent at ordinary temperature in asuperficial linear velocity of 5 cm/sec or less under atmosphericpressure.

As the pre-treatment cleaning agent, a cleaning agent composed of ametal hydroxide and/or a metal oxide is preferably used. For example,suitable as the pre-treatment cleaning agent is an acid gas-removingagent containing strontium hydroxide as a main component together withan organic binder and a hydroxide of alkaline earth metal other thanstrontium (Japanese Patent Application Laid-Open No. 9-99216). Examplesof the organic binders include polyvinyl alcohol, polyethylene glycol,polypropylene glycol, methyl cellulose and carboxymethyl cellulose.Examples of the hydroxides of alkaline earth metal other than strontiuminclude magnesium hydroxide, calcium hydroxide and barium hydroxide. Theamount of the organic binder is usually 0.1 to 40 parts by weight basedon 100 parts by weight of anhydrous strontium hydroxide. The amount ofthe hydroxide of alkaline earth metal other than strontium is usually0.05 to 1 mole per mole of strontium hydroxide. Examples of otherpre-treatment cleaning agents usable in the present invention include acleaning agent composed of zinc oxide, aluminum oxide and an alkalicompound (Japanese Patent Application Laid-Open No. 5-237324), acleaning agent composed mainly of strontium hydroxide and iron oxide(Japanese Patent Application Laid-Open No. 7-308538), and the cleaningagent prepared by adherently adding sodium formate to metal oxidecomposed mainly of copper oxide and manganese oxide, which is alsousable as the post-treatment cleaning agent as described above (JapanesePatent Application Laid-Open No.9-234337). Activated carbon isunsuitable as a component of the pre-treatment cleaning agent.

By combining the cleaning agent of the present invention with thepre-treatment cleaning agent, the cleaning of exhaust gases isefficiently performed even when the exhaust gases contain an oxidativehalogen-based gas in addition to an acidic halogen-based gas.

Further, by combining the cleaning agent of the present invention withboth the pre-treatment and post-treatment cleaning agents, a completecleaning of exhaust gases is achieved even when the exhaust gasescontain any kinds of halogen-based gases.

The use of the pre-treatment cleaning agent enables the simultaneousremoval of chlorine, hydrogen chloride, bromine and iodine in additionto highly oxidative gases such as fluorine and chlorine trifluoride.Even when the pre-treatment cleaning agent loses its cleaning capabilityby cleaning halogen-based gases other than fluorine or chlorinetrifluoride, the pre-treatment cleaning agent is still effective forcleaning fluorine and chlorine trifluoride because chlorine, bromine oriodine atoms already fixed thereon are replaced by fluorine atoms ofhighly reactive fluorine or chlorine trifluoride which is brought intocontact with the pre-treatment cleaning agent. Therefore, the cleaningcapability of the pre-treatment cleaning agent for fluorine or chlorinetrifluoride is maintained until the pre-treatment cleaning agent issaturated with fluorine atoms. Thus, fluorine or chlorine trifluoride isprevented from entering into the cleaning agent of the present inventionuntil the pre-treatment cleaning agent is saturated.

The cleaning method combinedly using the cleaning agent of the presentinvention, the pre-treatment cleaning agent and optionally thepost-treatment cleaning agent is especially useful for cleaninghalogen-containing exhaust gases containing at least one main gasselected from the group consisting of nitrogen, argon, helium andhydrogen, and at least one halogen-based gas selected from the groupconsisting of fluorine, chlorine trifluoride, chlorine, bromine, iodine,hydrogen fluoride, hydrogen chloride, hydrogen bromide, hydrogen iodide,boron trifluoride, boron trichloride, silicon tetrafluoride, silicontetrachloride, titanium tetrachloride, aluminum chloride, germaniumtetrafluoride and tungsten hexafluoride.

The optimum quantitative proportions of the pre-treatment cleaningagent, the post-treatment cleaning agent and the cleaning agent of thepresent invention may be easily determined based on flow rate and flowvelocity of exhaust gases to be treated, purging time, etc. Thesecleaning agents may be packed in separate columns respectively or may belaminated one over another in a single column.

When activated carbon adherently added with no chemical agent orconventionally known activated carbons adherently added with chemicalagents are used in place of the cleaning agent of the present invention,the amount of halogen-based gases desorbed from activated carbon becomeslarge, so that the amount of the post-treatment cleaning agent forfixing the desorbed halogen-based gases by chemical reactions must beincreased. This reduces the overall cleaning capability and increasesthe running cost.

The present invention will be described in more detail by reference tothe following examples. However, it should be noted that the followingexamples are illustrative and not intended to limit the inventionthereto.

EXAMPLES 1 TO 26

(Preparation of Cleaning Agent)

In the following preparation, 500 g of activated carbon produced byTakeda Yakuhin Kogyo Co., Ltd. under the trade name of “SHIRASAGI G2C10/20” were used.

A predetermined amount of alkali metal form ate or alkaline earth metalformate, or a predetermined amount of a mixture of a formate of alkalimetal or alkaline earth metal and a hydroxide of alkali metal oralkaline earth metal in a chemically equivalent ratio was dissolved in200 ml of water. The activated carbons were impregnated with theresultant solution. Then, the impregnated activated carbons were died at50° C. for 16 hours to obtain each cleaning agent.

(Measurement of Cleaning Capability)

Each cleaning agent thus prepared was packed into a SUS316L cylindricalcolumn having an inner diameter of 39.4 mm in a packed length of 100 mm,thereby preparing each cleaning column. Nitrogen gas containing 2,000ppm of halogen-based gas was continuously passed through the cleaningcolumn at 25° C. in a superficial linear velocity of 2.65 cm/sec underatmospheric pressure. During the continuous flowing of the gas, a partof effluent gas from an outlet of the column was sampled and introducedinto a gas-detecting tube (manufactured by Gastec Co., Ltd.) and a gasdetector (TG-XA manufactured by Bionics Kiki Co., Ltd.) to detect abreak-through point (point at which the concentration of halogen-basedgas exceeds 1 ppm). The amount of halogen-based gas adsorbed by thecleaning agent was calculated from the amount of the halogen-based gaspassed through the column until reaching the break-through point. Afterreaching the break-through point, nitrogen gas was allowed to passthrough the cleaning column in place of nitrogen gas containinghalogen-based gas, thereby purging the cleaning column for 24 hours.During the purging, the concentration of halogen-based gas in effluentnitrogen gas from the outlet was measured to determine the amount ofdesorbed halogen-based gas. The cleaning capability of the cleaningagent was defined as the value obtained by subtracting the measuredamount of desorbed halogen-based gas from the measured amount ofadsorbed halogen-based gas. The adsorbed amount, the desorbed amount andthe cleaning capability were represented by the volume of halogen-basedgas per liter of the cleaning agent (L/L cleaning agent). The volumes ofgases in the following Examples and Comparative Examples are those at25° C. under atmospheric pressure.

The results of cleaning tests on chlorine-containing test exhaust gasesare shown in Table 1. The results of cleaning tests on borontrichloride-containing test exhaust gases are shown in Table 3 and theresults of cleaning tests on test exhaust gases respectively containingdifferent halogen-based gases are shown in Table 5.

In the following Tables, “adhered amount” represents a weight percentageof a first component, i.e., alkali metal formate and alkaline earthmetal formate, based on the weight of a dried cleaning agent, or aweight percentage of a total of first and second components based on theweight of a dried cleaning agent when alkali metal hydroxide or alkalineearth metal hydroxide (second component) was adherently added inaddition to the first component.

Comparative Examples 1 to 26

The procedures of Examples 1-26 were repeated using the same activatedcarbons as used in Examples 1-26 to prepare cleaning agents shown inTable 2. The cleaning capability of each cleaning agent was measured inthe same manner as in Examples 1-26.

The results of cleaning tests on chlorine-containing test exhaust gasare shown in Table 2, and the results of cleaning tests on borontrichloride-containing test exhaust gas are shown in Table 4. In Table6, the results of cleaning tests of activated carbon adherently addedwith no chemical agent against test exhaust gases containing differenthalogen-based gases are shown. The results of cleaning tests ofactivated carbon adherently added with tetramethylammonium hydroxideagainst test exhaust gases containing different halogen-based gases areshown in Table 7.

TABLE 1 Adhered Components of Cleaning Agents First Second EquivalentAdhered amount Examples component component ratio (wt. %) 1 HCOONa None— 5 2 HCOONa None — 10 3 HCOONa None — 15 4 HCOONa NaOH 1:1 5 5 HCOONaNaOH 1:1 10 6 HCOONa NaOH 1:1 15 7 HCOONa KOH 1:1 10 8 HCOONa Ca(OH)₂1:1 10 9 HCOOK NaOH 1:1 10 10 (HCOO)₂Ca NaOH 1:1 10 Results of CleaningTests (halogen-based gas: chlorine) Adsorbed amount Desorbed amountCleaning capability Examples (L/L)* (L/L)* (L/L)* 1 25 8 17 2 26 7 19 322 4 18 4 31 10 21 5 30 8 22 6 25 5 20 7 29 9 20 8 28 9 19 9 28 10 18 1026 8 18 Note: *liter/liter of cleaning agent.

TABLE 2 Adhered Components of Cleaning Agents Comparative First SecondEquivalent Adhered Examples component component ratio amount (wt. %) 1None None — — 2 HCOONa None — 20 3 Na₃AlO₃ None — 10 4 [(CH₃)₄N]OH None— 10 5 NaOH None — 10 6 HCOONa NaOH 1:1 1 7 HCOONa NaOH 1:1 20 Resultsof Cleaning Tests (halogen-based gas: chlorine) Comparative Adsorbedamount Desorbed amount Cleaning capability Examples (L/L)* (L/L)* (L/L)*1 25 13 12 2 5 1 4 3 16 8 8 4 19 7 12 5 17 7 10 6 19 9 10 7 6 1 5 Note:*liter per liter of cleaning agent.

TABLE 3 Adhered Components of Cleaning Agents First Second EquivalentAdhered Examples component component ratio amount (wt. %) 11 HCOONa None— 5 12 HCOONa None — 10 13 HCOONa None — 15 14 HCOONa NaOH 1:1 5 15HCOONa NaOH 1:1 10 16 HCOONa NaOH 1:1 15 17 HCOONa KOH 1:1 10 18 HCOONaCa(OH)₂ 1:1 10 19 HCOOK NaOH 1:1 10 20 (HCOO)₂Ca NaOH 1:1 10 Results ofCleaning Tests (halogen-based gas: boron trichloride) Adsorbed amountDesorbed amount Cleaning capability Examples (L/L)* (L/L)* (L/L)* 11 207 13 12 20 7 13 13 19 6 13 14 20 5 15 15 21 4 17 16 19 3 16 17 20 5 1518 21 8 13 19 21 7 14 20 19 5 14 Note: *liter per liter of cleaningagent.

TABLE 4 Adhered Components of Cleaning Agents Comparative First SecondEquivalent Adhered Examples component component ratio amount (wt %) 8None None — 20 9 HCOONa None — 20 10 Na₃AlO₃ None — 10 11 [(CH₃)₄N]OHNone — 10 12 NaOH None — 10 13 HCOONa NaOH 1:1 1 14 HCOONa NaOH 1:1 20Results of Cleaning Tests (halogen-based gas: boron trichloride)Comparative Adsorbed amount Desorbed amount Cleaning capability Examples(L/L)* (L/L)* (L/L)* 8 20 12 8 9 5 1 4 10 15 8 7 11 16 7 9 12 16 7 9 1318 10 8 14 5 1 4 Note: *liter per liter of cleaning agent.

TABLE 5 Adhered Components of Cleaning Agents First Second EquivalentAdhered Examples component component ratio amount (wt. %) 21 HCOONa NaOH1:1 10 22 HCOONa NaOH 1:1 10 23 HCOONa NaOH 1:1 10 24 HCOONa NaOH 1:1 1025 HCOONa NaOH 1:1 10 26 HCOONa NaOH 1:1 10 Results of Cleaning TestsCleaning Halogen-based Adsorbed Desorbed capability Examples gas amount(L/L)* amount (L/L)* (L/L)* 21 HCl 52 11 41 22 BF₃ 32 7 25 23 WF₆ 23 320 24 SiF₄ 23 4 19 25 Br₂ 29 8 21 26 HBr 49 10 39 Note: *liter per literof cleaning agent

TABLE 6 Adbered Components of Cleaning Agents Comparative First SecondEquivalent Adhered Examples component component ratio amount (wt. %) 15None None — — 16 None None — — 17 None None — — 18 None None — — 19 NoneNone — — 20 None None — — Results of Cleaning Tests Cleaning ComparativeHalogen-based Adsorbed Desorbed *capability Examples gas amount (L/L)*amount (L/L)* (L/L)* 15 HCl 16 8 8 16 BF₃ 29 17 12 17 WF₆ 22 8 14 18SiF₄ 20 9 11 19 Br₂ 24 15 9 20 HBr 15 8 7 Note: *liter per liter ofcleaning agent.

TABLE 7 Adhered Components of Cleaning Agents Comparative First SecondEquivalent Adhered Examples component component ratio amount (wt. %) 21[(CH₃)₄N]OH None — 10 22 [(CH₃)₄N]OH None — 10 23 [(CH₃)₄N]OH None — 1024 [(CH₃)₄N]OH None — 10 25 [(CH₃)₄N]OH None — 10 26 [(CH₃)₄N]OH None —10 Results of Cleaning Tests Cleaining Comparative Halogen-basedAdsorbed Desorbed *capability Examples gas amount (L/L)* amount (L/L)*(L/L)* 21 HCl 39 15 24 22 BF₃ 28 12 16 23 WF₆ 22 7 15 24 SiF₄ 21 7 14 25Br₂ 25 14 11 26 HBr 38 14 24 Note: *liter per liter of cleaning agent.

EXAMPLE 27

Strontium hydroxide, calcium hydroxide and polyvinyl alcohol werekneaded in a weight ratio of 75:23:2, extruded and then dried asdescribed in Japanese Patent Application Laid-Open No. 9-99216, therebypreparing a cleaning agent. The cleaning agent was packed into acylindrical column of the same type as used in Examples 1-26 in a packedlength of 50 mm, thereby a first cleaning column (pre-treatment cleaningcolumn).

Separately, the cleaning agent prepared in Example 5 was packed into acylindrical column of the same type as used in Examples 1-26 in a packed680 mm, thereby producing a second cleaning column.

Cleaning tests were carried out at 25° C. under atmospheric pressure.More specifically, the first cleaning column and the second cleaningcolumn disposed on the downstream of the first cleaning column wereconnected tube. Nitrogen gas containing 2,000 ppm fluorine was allowedto flow from the first cleaning column at a superficial linear velocityof 2.65 cm/sec for 12 hours. Then, after nitrogen gas containing 4,000ppm chlorine was allowed to flow through the first and second cleaningcolumns at a superficial linear velocity of 2.65 for 8 hours, thecolumns were purged with nitrogen for 16 hours. These procedures werecycled repeatedly. During the above procedures, the time required untilthe concentration of halogen-based gas in the effluent gas from thesecond cleaning column exceeded 1 ppm was measured using a gas detector(“TG-XA” manufactured by Bionics Kiki Co., Ltd.), thereby calculatingthe amount of chlorine gas passed through the columns. As a result, theamount of chlorine gas passed through the columns was 41.4 liters andthe adsorbed amount was 50 liters per liter of the second cleaningagent.

After the cleaning test, the respective cleaning agents in the first andsecond cleaning columns were analyzed by an energy-dispersive X-raydiffractometer. The results showed that fluorine was detected only fromthe cleaning agent packed in the first cleaning column, and no fluorinewas detected from the cleaning agent packed in the second cleaningcolumn.

EXAMPLE 28

The same cleaning test as described in Example 27 was repeated exceptthat boron trichloride was used in place of chlorine. As a result, theamount of boron trichloride passed through the columns was 24.8 litersand the adsorbed amount was 30 liters per liter of the second cleaningagent.

Further, the results of energy-dispersive X-ray analysis showed thatfluorine was detected only from the cleaning agent packed in the firstcleaning column.

EXAMPLE 29

Hopcalite was adherently added with sodium formate in an amount of 20%by weight and dried at 50° C. as described in Japanese PatentApplication Laid-Open No. 9-234337 to prepare a cleaning agent. Thecleaning agent was packed into a cylindrical column of the same type asused in Examples 1-26 in a packed length of 120 mm, thereby producing athird cleaning column (post-treatment cleaning column). The same firstand second cleaning columns as used in Example 27 and the third cleaningcolumn were disposed in this order from the upstream side toward thedownstream side, and the cleaning test was conducted at 25° C. underatmospheric pressure.

Nitrogen gas containing 2,000 ppm of fluorine was allowed to flow fromthe first cleaning column at a superficial linear velocity of 2.65cm/sec for 12 hours. Then, after nitrogen gas containing 4,000 ppm ofchlorine and 4,000 ppm of boron trichloride was flowed at a superficiallinear velocity of 2.65 cm/sec for 8 hours, the columns were purged withnitrogen for 16 hours. These procedures were cycled repeatedly. Duringthe above procedures, a part of the effluent gas was sampled from asampling port provided between the second and third cleaning columns tomeasure the time required until the concentration of the halogen-basedgas in the effluent gas exceeded 1 ppm (break-through point) by a gasdetector (TG-XA manufactured by Bionics Kiki Co., Ltd.). The amount ofthe halogen-based gas flowed through the columns was calculated from themeasured value.

After reaching the break-through point, the columns were purged withnitrogen at a superficial linear velocity of 2.65 cm/sec, and the timerequired until the concentration of the halogen-based gas in theeffluent gas from the third cleaning column exceeded 1 ppm was measuredby a gas detector (TG-XA manufactured by Bionics Kiki Co., Ltd.).

As a result, the amounts flowed through the columns until reaching thebreak-through point of the second cleaning column were 16.6 liters foreach of chlorine and boron trichloride, and the total amount of adsorbedchloride and boron trichloride was 40 liters per liter of the secondcleaning agent. Further, the time required until chlorine gas was firstdetected in the effluent gas from the third cleaning column was 45 hoursafter initiating the nitrogen purge.

Comparative Example 27

The same procedures as described in Example 29 were repeated except thatthe cleaning agent in the second cleaning column was replaced byactivated carbon added with no chemical agent.

As a result, the amounts passed through the columns until reaching thebreak-through point of the second cleaning column were 13.5 liters foreach of chlorine and boron trichloride, and the total amount of adsorbedchlorine and boron trichloride was 32 liters per liter of activatedcarbon. Further, the time required until chlorine gas was first detectedin the effluent gas from the third cleaning column was 18 hours afterinitiating the nitrogen purge.

EXAMPLE 30

The cleaning agent prepared in Example 5 was packed into a cylindricalcolumn of the same type as used in Examples 1-26 in a packed length of680 mm, thereby producing a first cleaning column. The hopcaliteprepared in Example 29 was packed into another cylindrical column in apacked length of 120 mm, thereby producing a second cleaning column(post-treatment cleaning column). The second cleaning column wasdisposed on the downstream of the first cleaning column, and connectedthereto through a tube provided with a sampling port.

After nitrogen gas containing 4,000 ppm of chlorine and 4,000 ppm ofboron trichloride was passed through the first and second columns at asuperficial linear velocity of 2.65 cm/sec for 8 hours, the columns werepurged with nitrogen for 16 hours. These procedures were cycledrepeatedly. During the above procedures, a part of the effluent gas fromthe first cleaning column was sampled from the sampling port to measurethe time required until the concentration of halogen-based gas exceeded1 ppm (break-through point) by a gas detector (TG-XA manufactured byBionics Kiki Co., Ltd.). The amount of halogen-based gas flowed throughthe columns was calculated from the measured value.

After reaching the break-through point, the columns were purged withnitrogen at a superficial linear velocity of 2.65 cm/sec, and the timerequired until the concentration of halogen-based gas in the effluentgas from the second cleaning column exceeded 1 ppm was measured by a gasdetector (TG-XA manufactured by Bionics Kiki Co., Ltd.).

As a result, the amounts flowed through the columns until reaching thebreak-through point were 16.6 liters for each of chlorine and borontrichloride, and the total amount of adsorbed chlorine and borontrichloride was 40 liters per liter of the first cleaning agent.Further, the time required until chlorine gas was first detected in theeffluent gas from the second cleaning column was 45 hours afterinitiating the nitrogen purge.

EXAMPLES 31 to 32

A cleaning agent composed of activated carbon adherently added withalkaline earth metal formate (Example 31) and a cleaning agent composedof activated carbon adherently added with alkali metal formate andalkaline earth metal formate in an amount of 10% by weight in total atan equivalent ratio of 1:1 (Example 32) were prepared in the same manneras in Examples 1-26.

The cleaning capability of the cleaning agents were measured in the samemanner as in Examples 1-26. The results are shown in Table 8.

TABLE 8 Adhered Components of Cleaning Agents First Second EquivalentAdhered Examples component component ratio (wt. %) 31 (HCOO)₂Ca None — 532 HCOONa None — 10 (HCOO)₂Ca Results of Cleaning Tests CleaningHalogen-based Adsorbed Desorbed *capability Examples gas amount (L/L)*amount (L/L)* (L/L)* 31 Cl₂ 25 8 17 32 Cl₂ 27 8 19 Note: *liter perliter of cleaning agent.

INDUSTRIAL APPLICABILITY

The cleaning agents and the cleaning processes according to the presentinvention have the following excellent effects, and are useful forcleaning exhaust gases containing noxious halogen-based gases frometching process of semiconductor production.

(1) Large cleaning capability is attained because the desorbed amount ofhalogen-based gases from the cleaning agent is considerably reduced.

(2) The cleaning agent is prepared at low production cost from activatedcarbon and formate, or activated carbon, formate and hydroxide.

(3) Halogen-based gases are completely removed by disposing a cleaningagent prepared by adherently adding sodium formate to a metal oxide onthe downstream side of the cleaning agent of the present invention.

(4) Cleaning of halogen-based gases containing fluorine or chlorinetrifluoride is safely performed by disposing a cleaning agent composedof a metal oxide and/or a metal hydroxide on the upstream side of thecleaning agent according to the present invention.

(5) Exhaust gases containing any kinds of halogen-based gases iscompletely and safely cleaned with great cleaning efficiency bydisposing a cleaning agent composed of a metal oxide and/or a metalhydroxide on the upstream side of the cleaning agent of the presentinvention, and disposing a cleaning agent prepared by adherently addingsodium formate to a metal oxide on the downstream side of the cleaningagent of the present invention.

What is claimed is:
 1. A process for cleaning a halogen-based gascontaining exhaust gas, comprising: contacting an exhaust gas containinga halogen-based gas as noxious component with a cleaning agentcomprising activated carbon adherently added with an alkali metalformate and/or an alkaline earth metal formate; wherein thehalogen-based gas is at least one gas selected from the group consistingof fluorine, chlorine, bromine, iodine, hydrogen fluoride, hydrogenbromide, hydrogen iodide, chlorine trifluoride, boron trifluoride, borontrichloride, silicon tetrafluoride, silicon tetrachloride, titaniumtetrachloride, aluminum chloride, germanium tetrafluoride and tungstenhexafluoride.
 2. The process according to claim 1, wherein an adheredamount of the alkali metal formate and/or alkaline earth metal formateis 3 to 18% by weight (dry basis) based on the weight of the cleaningagent.
 3. A process for cleaning a halogen-based gas containing exhaustgas, comprising: contacting an exhaust gas containing a halogen-basedgas as noxious component with a cleaning agent comprising activatedcarbon adherently added with an alkali metal hydroxide and/or analkaline earth metal hydroxide in addition to an alkali metal formateand/or an alkaline earth metal formate; wherein the halogen-based gas isat least one gas selected from the group consisting of fluorine,chlorine, bromine, iodine, hydrogen fluoride, hydrogen bromide, hydrogeniodide, chlorine trifluoride, boron trifluoride, boron trichloride,silicon tetrafluoride, silicon tetrachloride, titanium tetrachloride,aluminum chloride, germanium tetrafluoride and tungsten hexafluoride. 4.The process according to claim 3, wherein a total amount of the alkalimetal formate and/or alkaline earth metal formate and the alkali metalhydroxide and/or alkaline earth metal hydroxide is 3 to 18% by weight(dry basis) based on the weight of the cleaning agent.
 5. The processaccording to claim 3, wherein a ratio of equivalent amount of the alkalimetal formate and/or alkaline earth metal formate to the alkali metalhydroxide and/or alkaline earth metal hydroxide is 1:0.1-3.0.
 6. Theprocess according to claims 1 or 3, further comprising a post-treatmentstep of contacting the exhaust gas containing the halogen-based gas witha cleaning agent comprising a metal oxide adherently added with sodiumformate.
 7. The process according to claims 1 or 3, further comprising apre-treatment step of contacting the exhaust gas containing thehalogen-based gas with a cleaning agent comprising a metal oxide and/ora metal hydroxide.
 8. The process according to claims 1 or 3, furthercomprising a pre-treatment step of contacting the exhaust gas containingthe halogen-based gas with a cleaning agent comprising a metal oxideand/or a metal hydroxide; and a post-treatment step of contacting theexhaust gas containing the halogen-based gas with a cleaning agentcomprising a metal oxide adherently added with sodium formate.
 9. Acleaning agent for a halogen-based gas containing exhaust gas,comprising: activated carbon adherently added with an alkali metalformate and/or an alkaline earth metal formate; wherein an adheredamount of the alkali metal formate and/or the alkaline earth metalformate is 3 to 18% by weight based on the weight of the dry cleaningagent; wherein said exhaust gas contains the halogen-based gas as anoxious component; and wherein said halogen-based gas is at least onegas selected from the group consisting of fluorine, chlorine, bromine,iodine, hydrogen fluoride, hydrogen bromide, hydrogen iodide, chlorinetrifluoride, boron trifluoride, boron trichloride, silicontetrafluoride, silicon tetrachloride, titanium tetrachloride, aluminumchloride, germanium tetrafluoride and tungsten hexafluoride.
 10. Thecleaning agent according to claim 9, wherein said activated carbon isselected from the group consisting of a coal activated carbon, acharcoal activated carbon and a coconut shell activated carbon.
 11. Thecleaning agent according to claim 9, wherein said alkali formate issodium formate, potassium formate or a mixture thereof.
 12. The cleaningagent according to claim 9, wherein said alkaline earth metal formate iscalcium formate, magnesium formate, barium formate, strontium formate ora mixture thereof.
 13. A cleaning agent for a halogen-based gascontaining exhaust gas, comprising: activated carbon adherently addedwith an alkali metal hydroxide and/or an alkaline earth metal hydroxidein addition to an alkali metal formate and/or an alkaline earth metalformate; wherein a total adhered amount of the alkali metal formateand/or the alkaline earth metal formate and the alkali metal hydroxideand/or the alkaline earth metal hydroxide is 3 to 18% by weight based onthe weight of the dry cleaning agent; wherein said exhaust gas containsthe halogen-based gas as a noxious component; and wherein thehalogen-based gas is at least one gas selected from the group consistingof fluorine, chlorine, bromine, iodine, hydrogen fluoride, hydrogenbromide, hydrogen iodide, chlorine trifluoride, boron trifluoride, borontrichloride, silicon tetrafluoride, silicon tetrachloride, titaniumtetrachloride, aluminum chloride, germanium tetrafluoride and tungstenhexafluoride.
 14. The cleaning agent according to claim 13, wherein aratio of equivalent amount of the alkali metal formate and/or alkalineearth metal formate to the alkali metal hydroxide and/or alkaline earthmetal hydroxide is 1:0.1-3.0.
 15. The cleaning agent according to claim13, wherein said alkali metal hydroxide is selected from the groupconsisting of lithium hydroxide, sodium hydroxide, potassium hydroxide,and a mixture thereof.
 16. The cleaning agent according to claim 13,wherein said alkaline earth metal hydroxide is selected from the groupconsisting of calcium hydroxide, barium hydroxide, strontium hydroxide,magnesium hydroxide and a mixture thereof.
 17. The cleaning agentaccording to claim 13, wherein a ratio of the formate to the hydroxideis 1:0.1 to 1:3.0.